What Does The Florida Red Tide Do To Stone Crabs

Concerns over stone crab following months of heavy red tide in the waters around Florida

  • Stone crab are only legal to harvest between October 15 and May 15

  • One study conducted suggests the red tide has a significant impact on stone crabs

  • Moderately high levels of red tide showed a drop in reflexes appetite and mortality

Monday marked the start of harvest season for stone crabs. Stone crabs are only legal to harvest between October 15 and May 15. However, many are concerned about this year’s stone crab season following the harsh red tides that plagued the area over the last several months.

How exactly does the red tide affect stone crabs? That is a question that lacks the research to give a definite answer. The little research that does exist appears to show they do react to the presence of Karenia brevis — the marine dinoflagellate responsible for causing the red tide.

Preliminary research shows the red tide puts stress on sublegal stone crabs — adolescent crabs with claws that are not long enough to legally harvest yet — and possibly even cause death.

Mote Marine Laboratory scientists, along with the Florida Fish and Wildlife Conservation Commission’s Fish and Wildlife Research Institute (FWRI), conducted a study that indicated a “strong need for continued studies.”

The inspiration for the study came from Mote Postdoctoral Research Fellow Dr. Phil Gravinese attending the state’s Commercial Stone Crab Advisory Panel meeting. Gravinese heard questions from stone crab fishers regarding the impact of the red tide on their traps.

I was at the state’s Commercial Stone Crab Advisory Panel meeting, and some of the fishermen from the Gulf Coast were asking why their traps were relatively empty during six months of red tide.

Also, some reported catching crabs that looked lethargic. The scientists present at the meeting didn’t have a clear answer about whether red tide was affecting the crabs, so we designed an initial study to learn more.

FWRI scientists referred to another study conducted on stone crabs not related to the red tide. The study looked at certain stone crab reflexes as a way to judge the chance of survival. FWRI developed a tool known as “reflex action mortality predictor” (RAMP) to gather data on how well stone crab survive trapping and handling by fishers.

In 2017, project partners brought 36 stone crabs from New Pass, Sarasota, to Mote’s City Island research campus. The stone crabs were placed in three different environments. Researchers put 12 crabs in tanks filled with seawater, 12 in tanks with high levels of K. brevis and 12 in low levels of K. brevis.

All 36 stone crabs used in the study were sublegal since they represent the following year’s harvest and the data can be used for management of the fishery. Dr. Vince Lovko, Phytoplankton Ecology Program Manager at Mote and a co-author on the study explained the K. brevis was similar to what would be found during a “moderately high bloom” along the west coast of Florida.

For this study, we used concentrations of Karenia brevis similar to what would be found along the west coast of Florida during a moderately high bloom.

It’s quite possible for stone crabs in this area to encounter concentrations of red tide ranging from ‘low,’ meaning elevated above the sparse ‘background’ concentration normally present in the Gulf of Mexico, to levels many times above what was used in this study.

For nine days researchers monitored the stone crabs in their environments. Researchers looked for signs of stress, such as decreased feeding and reflexes — “retracting an eye stalk or leg, closing the mouth and other movements in response to touch or pressure.”

The stone crabs introduced to the red tide showed the worst signs of stress in appetite, reflexes, and mortality. The stone crabs placed in the seawater suffered a 10% loss in their reflex scores, making them the least stressed of the three groups.

In the tanks with low levels of red tide, the stone crabs had a 32% drop in their reflex score. This group also experienced a 43% decrease in appetite and a 25% drop in their ability to survive. This leads researchers to believe the group was more stressed than the group in seawater.

Stone crabs introduced to high levels of red tide had their reflex score drop by 52%. This group also ate 67% less food and had a 42% drop in their survival rate. These results obviously lead researchers to believe the substantial differences between the control group show K. brevis does have an impact on stone crabs.

The study leads researchers to believe that exposure to the red tide could make juvenile stone crabs more vulnerable to predators. However, it is also thought that adults may have a higher chance of being able to escape the red tide in the wild if it is a moderate or low level. Juvenile stone crabs can travel a significant lesser distance than adults, but if you have a heavy red tide that lasts for months, it is hard to imagine even an adult stone crab could escape its reach.

Barefoot Beach in Bonita Springs is one of the many beaches that have witnessed stone crabs washing up and dying on shore. The death of these stone crabs leads to yet another concern of the aftermath of these heavy red tides, the economic impact on businesses in the area.

Florida is responsible for 80% of the stone crabs that are served in America. Fishers in the area have concerns about how the stone crab season is going to turnout after months of red tide plaguing a large portion of Florida. Researchers have been trying to determine why there has been a continuing decrease in catching stone crabs since 2000.

Gravinese has other studies to learn more about how stone crabs react to the red tide. One study examines stone crabs that have died from K. brevis to see if it is “in the muscle, the viscera — the gut tract — or something else.”

Another study has 10-12 stone crabs eating clams exposed to the red tide in seawater tanks with K. brevis present while the other 10-12 eat healthy clams in clean water. The study hopes to determine if stone crabs die from the food they eat during a red tide.

If you would like to help us with our documentary on the red tide, you can click here!

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About Meko Haze

Meko Haze is an independent journalist by day... and an independent journalist by night.

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